In order to keep pace with the continuing trend towards portabilization, miniaturization, and higher functionalization of a variety of electronic devices and instruments with development of the Information-Electronic Industry, higher capacity, smaller and lighter lithium secondary batteries are increasingly in demand. Recently, with diversification of functions of the electronic devices and instruments, there is also a strong need for realization of higher capacity, functionality, and performance of lithium secondary batteries, as a main power source. In addition, as the temperature range of lithium secondary batteries in use is also further extended, environmental conditions to which batteries are applied become more severe such that maintenance of initial performance without degradation in battery performance even when being maintained at higher temperatures of more than 80° C. for a prolonged period of time is required.
However, when material such as LiCoO2, LiMn2O4 or the like is used as a cathode active material for the lithium secondary battery, reaction between electrolyte and electrode surface leads to poor high-temperature characteristics, such as swelling and bulking in storage at a higher temperature of more than 80° C. Additionally, when charge/discharge cycles are repeatedly performed at the higher temperature, the electrochemical reaction continuously occurring at a cathode or anode surface contributes to production of decomposed by-products of the electrolyte and gas in the battery, thereby severely swelling the battery.
A variety of study and research has been made to solve those problems. Korean Patent Laid-open Publication No. 2003-0057321 discloses a technique of forming a protective coating film using electrolyte additives such as vinylene carbonate, alkenylethylene carbonate and the like, wherein the electrolyte additives are used to inhibit electrolyte decomposition reaction of the cathode at higher temperatures, such that these additives are first decomposed at cathode potential, thereby forming the protective film. However, although such additives may effectively inhibit battery swelling at higher temperatures, the additives may result in reduction of battery capacity or deterioration of battery life characteristics. For these reasons, various kinds of additives should be utilized to improve one functionality of the battery, when such electrolyte additives are used.
In addition, Korean Patent Laid-open Publication No. 2001-35700 and Japanese Patent Laid-Open Publication No. 1998-255389 disclose techniques of improving battery life characteristics and high temperature storage characteristics by addition of metal oxides or metal hydroxides to an electrode (cathode). However, metal oxides or metal hydroxides are electrically non-conductive and thus, when they are used as additives, interfere with electrical flow in the electrodes, thereby resulting in decreased high rate discharge characteristics of the battery, as well as increased resistance thereof leading to lowering of battery life characteristics.
Japanese Patent Laid-Open Publication Nos. 2003-86174 and 1997-22733 disclose a method of coating the cathode active material with an electro-conductive material such as conductive material using a mechanofusion apparatus and a binder. However, due to integration between the cathode active material and the conductive material in this method, it may be possible to improve battery capacity or diminish electrical resistance, thereby improving high rate discharge characteristics, but the conductive material coated on the surface of the cathode active material rather hampers migration of lithium ions, resulting in deterioration of battery performance.
Therefore, there is a strong need for an electrode additive that facilitates excellent high temperature storage performance of the lithium secondary battery and simultaneously does not inhibit high rate discharge and other battery performance characteristics.